Question About Parasite Medication

Hi. I bought Tetra Parasite Guard to treat anchor worms for my guppies. The directions say to add one tablet for each ten gallons of aquarium water, but does that work for how much water the tank can hold, or only how much water is in there? Because mine is a 10 gallon tank, but I don't know how much is in there. Thanks!

Probably not far. I have about 10 pounds of gravel in the tank, one big decoration, two small decorations, two kinda small bits of Cryptocoryne Wendtii, a Top Fin Power 10 HOB filter, a seaweed clip, a thermometer, a small heater, and a small air stone in there. And 3 adult male guppies.